Differentiation of human HL60 cells (established from a patient with acute progranulocytic leukemia) and mouse peritoneal macrophages is being induced by a variety of agents. Changes in the differentiation status of these cells is associated with changes in both the overall amount of nuclease activity and the numbers of activities being expressed. The changes in the macrophages are being followed in normal mice and in mice that have genetic defects in their immune system. The effect of the lymphokine interferon upon differentiation and nuclease expression is also being investigated.